Various kinds of pressure sensitive adhesive tapes for skin have heretofore been developed for protection of the affected skin parts and operated sites, and percutaneous absorption of drugs. A pressure sensitive adhesive tape for skin generally has a structure that a pressure sensitive adhesive layer optionally containing a drug is formed on one side of a flexible base material.
Such a pressure sensitive adhesive tape for skin is required to (1) have good conformability to skin that permits conforming to the shape and movement of a skin surface applied, (2) prevent moisture by sweating from remaining therein so as to have excellent moisture permeability, (3) give neither pain nor separation of horny layer upon its peeling, (4) be able to retain tackiness to skin over a long period of time, and (5) cause none of dermal diseases such as rash, itch and erythema.
Since the adhesive tape is often stuck on the skin surface over a long period of time, however, it has involved a problem that troubles such as skin irritation always follow the skin surface covered therewith as side effects.
Various proposals have heretofore been made for lessening the troubles such as skin irritation. For example, there have been proposed (1) a method in which a crosslinked pressure sensitive adhesive layer comprising an acrylic ester polymer and a liquid component compatible with the polymer is formed on a base material, thereby lessening separation of corneocytes upon peeling (Japanese Patent Application Laid-Open No. 65224/1993), (2) a method in which moisture permeability is imparted to a base material and a pressure sensitive adhesive layer, thereby lightening dermal diseases and skin irritation (Japanese Patent Application Laid-Open Nos. 279251/1993, 157423/1995 and 275340/1995), (3) a method in which a base material composed of a water-absorbing polymer and a pressure sensitive adhesive containing a water-absorbing substance are used, thereby lightening skin irritation and separation of horny layer (Japanese Patent Application Laid-Open Nos. 16541/1994 and 206710/1995), (4) a method in which the amount of a monomer remaining in a pressure sensitive adhesive is reduced, thereby preventing the development of irritation, rash, itch, erythema, etc. (Japanese Patent Application Laid-Open No. 131022/1993), and (5) a method in which a stretchable base material is used, thereby imparting conformability to skin (Japanese Patent Application Laid-Open Nos. 138152/1995 and 117268/1996).
However, various factors are complicatedly entangled on the development of skin irritation by sticking of a pressure sensitive adhesive tape for skin, so that the mere improvement in a single factor or two factors does not result in a full solution of the problem. For example, when a measure to lighten skin irritation is adopted, such problems as tackiness and conformability to skin are deteriorated are easy to arise, and so it has been difficult to suitably balance the various characteristics with one another.
On the other hand, for example, a polyethylene film, ethylene-methyl methacrylate copolymer (EMMA) film, polyvinyl chloride film, polyurethane film, polyester elastomer film, polyvinyl alcohol film, microporous polyolefin film, nonwoven polyurethane fabric, nonwoven polyester elastomer fabric or the like is used as a base material used in a pressure sensitive adhesive tape for skin.
Among these base materials, films themselves such as the polyethylene film, EMMA film and polyvinyl chloride film do not have moisture permeability, and so holes are made therein by a perforator to impart the moisture permeability. However, such a film involves a problem that satisfactory moisture permeability is not given because the rate of holes bored by the perforation is at most several percent, and moreover a distance between individual holes is great and a portion, in which no hole is bored, has no moisture permeability.
The polyurethane film, polyester elastomer film, polyvinyl alcohol film, nonwoven polyurethane fabric, nonwoven polyester elastomer fabric, etc. involve a problem that they are too flexible in addition to their high prices. A pressure sensitive adhesive tape for skin, the base material of which is too flexible, wrinkles upon its application to a skin surface, and so its handling is difficult. Besides, the moisture permeability of the polyurethane film and polyvinyl alcohol film is markedly deteriorated when their thickness is increased to about 40 .mu.m, and they scarcely exhibit the moisture permeability when their thickness reaches 60 .mu.m or greater.
Some of microporous polyolefin films are known to have excellent moisture permeability as demonstrated by their water-vapor transmission rate of at least 2,500 g/m.sup.2.multidot.24 h, and they have good conformability to skin and are relatively cheap. Although many of the conventional microporous polyolefin films have an air permeability of 500 sec/100 cc or higher and are also small in film thickness and are hence suitable for use in paper diapers and sanitary products, however, they have been not satisfactory for base materials of pressure sensitive adhesive tapes for skin. An investigation by the present inventors have revealed that when a pressure sensitive adhesive layer is formed on the surface of a microporous polyolefin film having an air permeability of 500 sec/100 cc or higher, the water-vapor transmission rate thereof is markedly lowered, and so any pressure sensitive adhesive tape excellent in moisture permeability cannot be provided.
In order that a pressure sensitive adhesive tape for skin is hard to be steamed up, scarcely causes corneum separation and exhibits stable adhesive strength, it desirably has a water-vapor transmission rate of generally 500 g/m.sup.2.multidot.24 h or higher, preferably 700 g/m.sup.2.multidot.24 h or higher. When the microporous film described above is used as a base material, however, any pressure sensitive adhesive tape having satisfactory water-vapor transmission rate cannot be obtained. In addition, when the thickness of the microporous polyolefin film is too small, such a film involves a problem that it is too flexible as a base material. When the thickness thereof is made great, the water-vapor transmission rate thereof is further lowered when it is used in a pressure sensitive adhesive tape